Actually, I'd been considering the same thing with my 6th graders (my friend is a teacher back in Canada) ... but it takes a lot of coordination, so I'm going to leave that idea for next September, when the new school year starts back home. I'm going to pair it with a quick lesson about Canada and Canadian schools (just fun things like show them pictures of Canadian schools and classrooms, pictures of school uniforms, tell them that the school year is different, talk about the kind of afterschool clubs they have, etc.)
I was planning to structure it a little bit because my kids are low level. So what I would do is each week in the summer we would write a paragraph with a topic (my family, my house, a day in my life, my school, about Korea, questions about Canada, etc). They could combine it at the end into one letter that we'll send in Sept-Oct, and bring pictures from home to add to their letters.
I also thought a fun thing to do might be to do a class video message as well, where the kids could film themselves doing a tour of their school, and explaining what school life is like in Korea. Basically, each kid can just say a sentence or two. I thought they could teach a little bit of Korean in the video, too.
When they get letters back from Canada, we're going to do a translation activity. Then the kids will do a presentation, where the kids read the letters they received to the class and "introduce" their pen pal. Then hopefully we can watch a video message that we get back from the other school together.
I'm really hoping I can make this activity work, as my school is low-income, and most of the kids have never even been to Jeju before, let alone another country.